The disclosure of Japanese Patent Application No. 2001-192331 filed on Jun. 26, 2001 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to a technology for reducing or eliminating suspended particulates in exhaust gas produced from an internal combustion engine.
2. Description of the Related Art
The exhaust gas from internal combustion engines and, particularly, diesel engines, contains suspended particulates, such as black smoke (soot) and the like. It is strongly demanded that the total amount of carbon-containing suspended particulates emitted into the atmosphere be reduced in order to prevent further air pollution. A like demand exists regarding generally termed direct injection type gasoline engines in which gasoline is injected directly into a combustion chamber, because carbon-containing suspended particulates may be discharged together with exhaust gas depending on the engine operation conditions.
One proposed technology for reducing the carbon-containing suspended particulates emitted from an internal combustion engine and, particularly, a diesel engine, is a technology that removes suspended particulates from exhaust gas by using a heat-resistant filter loaded with an oxidation catalyst (Japanese Examined Patent Application Publication No. 7-106290). In this technology, suspended particulates in exhaust gas are collected by the filter, and the collected particulates are burned in exhaust gas at relatively low temperature (typically, 350xc2x0 C. to 400xc2x0 C.) due to the effect of the oxidation catalyst. Thus, by collecting and burning carbon-containing particulates in exhaust gas, the amount of suspended particulates released into the atmosphere can be considerably reduced.
However, in some cases, the temperature of exhaust gas becomes lower than the temperature (350xc2x0 C. to 400xc2x0 C.) that allows combustion of collected carbon-containing particulates. Therefore, during use of a filter, particulates deposit on the filter, thus giving rise to the problem of filter clogging. Specifically, under a condition that the exhaust gas temperature is lower than the temperature that allows combustion of collected carbon-containing particulates, suspended particulates in exhaust gas deposit on the filter. If the exhaust gas temperature sufficiently rises, combustion of carbon-containing particulates is resumed. However, some particulates on the filter may remain unburned if a great amount of particulates has deposited, and therefore needs a long time for complete combustion thereof. Furthermore, it is known that the carbon-containing particulates deposited on the filter gradually become less apt to burn. As incombustible particulates cover the oxidation catalyst, it becomes difficult to burn carbon-containing particulates on the filter. As the collected particulates cannot be appropriately processed, the filter becomes clogged in some cases.
It is an object of the invention to provide a technology capable of effectively reducing carbon-containing particulates suspended in exhaust gas without allowing a filter to be clogged.
A first aspect of the invention is an emission control apparatus for reducing carbon-containing particulates contained in an exhaust gas from an internal combustion engine. The emission control apparatus includes a heat-resistant filter provided in a passage of the exhaust gas for collecting the carbon-containing particulates and controlling the exhaust gas through combustion of the particulates collected, a temperature increase need determining device that determines whether to increase a temperature of the heat-resistant filter, and a dynamic pressure increasing device that increases a dynamic pressure of the exhaust gas on the heat-resistant filter if it is determined that the temperature of the heat-resistant filter is to be increased.
The emission control method of the invention corresponding to the above-described emission control apparatus is an emission control method for reducing carbon-containing particulates contained in an exhaust gas from an internal combustion engine. The method includes the steps of: collecting and burning the carbon-containing particulates by using a heat-resistant filter provided in a passage of the exhaust gas; determining whether to increase a temperature of the heat-resistant filter; and increasing a dynamic pressure on the exhaust gas on the heat-resistant filter if it is determined that the temperature of the heat-resistant filter is to be increased.
The invention has been accomplished based on consideration of a phenomenon found by the inventors in which the temperature of a heat-resistant filter provided in an exhaust passage rises if the dynamic pressure of exhaust gas discharged from an internal combustion engine acts on the heat-resistant filter. As a preparation for description of the operation and advantages of the invention, this newly found phenomenon will be briefly described.
FIGS. 19A and 19B are diagrams conceptually illustrating the phenomenon in which the dynamic pressure of exhaust gas acting on the heat-resistant filter increases the filter temperature. FIG. 19A conceptually illustrates a laboratory device in which a heat-resistant filter E is disposed in an exhaust pipe of an internal combustion engine A (representatively, a diesel engine). The internal combustion engine A draws in air from an intake pipe B, and burns fuel within a combustion chamber C, and discharges exhaust gas via an exhaust pipe D. Carbon-containing suspended particulates contained in exhaust gas, such as sooth and the like, are collected by the heat-resistant filter E provided in the exhaust pipe D. The temperature Tg of exhaust gas flowing into the heat-resistant filter E, and the temperature Tf of the heat-resistant filter can be measured.
Using the foregoing device, the temperature Tg of exhaust gas flowing into the heat-resistant filter and the filter temperature Tf were measured while the operation condition of the internal combustion engine A was changed. The filter temperature Tf was found to always exhibit a higher value than the inlet exhaust gas temperature Tg. Therefore, the inlet exhaust gas temperature Tg and the amount of increase of the filter temperature Tf from the inlet exhaust gas temperature Tg (dT=Tfxe2x88x92Tg) were determined while the exhaust gas temperature was changed with other factors, such as the amount of flow of exhaust gas and the like, being kept fixed as much as possible. Results are indicated in FIG. 19B.
As indicated in FIG. 19B, the amount of increase dT of the filter temperature exhibits a tendency of substantially linearly increasing with increases in the inlet exhaust gas temperature Tg. Therefore, it is assumed that the phenomenon in which the filter temperature Tf becomes higher than the inlet exhaust gas temperature Tg is based on a mechanism described below.
That is, the heat-resistant filter E has a gas flow resistance. Therefore, when exhaust gas passes through the heat-resistant filter E at great flow speed, exhaust gas is impeded by the heat-resistant filter E, so that a portion of the speed of exhaust gas is converted into pressure. This pressure increase is expressed as dP. According to teaching in thermodynamics, three variables of pressure P, temperature T and specific volume v satisfy the following relationship:
Pxc2x7v=Rxc2x7Txe2x80x83xe2x80x83(1) 
where R is a gas constant. Therefore, if the pressure P increases by dP due to impediment of flow of exhaust gas by the heat-resistant filter E, the exhaust gas temperature also increases by dT so as to satisfy equation (1). That is, the phenomenon in which the filter temperature Tf is always higher than the inlet exhaust gas temperature Tg is considered to be based on the following mechanism. That is, exhaust gas is compressed in the heat-resistant filter due to the dynamic pressure, so that the gas temperature correspondingly rises and the exhaust gas with the increased temperature warms the filter. Therefore, the inlet exhaust gas temperature Tg is always higher than the inlet exhaust gas temperature Tg.
In order to verify the validity of the mechanism, the following analysis was conducted based on measurement results as indicated in FIG. 19B. From the equation (1), an exhaust gas pressure Pg at an inlet of the heat-resistant filter E and the inlet exhaust gas temperature Tg satisfy the following equation:
Pgxc2x7v=Rxc2x7Tgxe2x80x83xe2x80x83(2) 
If the pressure and the temperature increase by dP and dT due to impediment of flow of exhaust gas by the heat-resistant filter E, the following equation holds based on the equation (1).
(Pg+dP)xc2x7v=Rxc2x7(Tg+dT)xe2x80x83xe2x80x83(3) 
By combining the equations (2) and (3), the following equation is obtained.
dT=(Tgxc2x7dP)/Pgxe2x80x83xe2x80x83(4) 
According to the equation (4), the temperature increase dT should be proportional to the temperature of exhaust gas flowing into the filter (inlet exhaust gas temperature Tg), which agrees with the measurement results indicated in FIG. 19B. That is, the measurement results in FIG. 19B support the validity of the speculated mechanism described above. The phenomenon in which the filter temperature Tf is always higher than the inlet exhaust gas temperature Tg measured at the inlet of the heat-resistant filter is considered to be based on a temperature increase of exhaust gas caused by compression of the exhaust gas that occurs when exhaust gas from the internal combustion engine passes through the heat-resistant filter.
As is apparent from the equation (4), the temperature increase dT increases with increases in the exhaust gas temperature Tg. If the flow speed of exhaust gas flowing into the heat-resistant filter increases, the pressure increase dP corresponding increases, and therefore, the temperature increase dT of the filter also increases. In general, the temperature of exhaust gas from an internal combustion engine decreases as exhaust gas flows through the exhaust pipe. As for the flow speed, when discharged from the internal combustion engine, exhaust gas instantaneously jets out, and forms a pulse-like flow with a great flow speed. However, as the exhaust gas passes through the exhaust pipe, the pulse-like flows average out, and the flow speed decreases. Therefore, if the location of the heat-resistant filter is closer to the internal combustion engine (for example, the heat-resistant filter is disposed in the exhaust port), the temperature and the flow speed of exhaust gas become greater, so that the filter temperature increase dT increases. Hence, the position of the heat-resistant filter closer to the engine is considered preferable.
The present invention has been accomplished based on the above-described newly found phenomenon and the considerations regarding the phenomenon. In the invention, carbon-containing suspended particulates in exhaust gas are collected by a heat-resistant filter. It is determined whether to increase the temperature of the heat-resistant filter. If it is determined that the filter temperature is to be increased, the dynamic pressure on the heat-resistant filter is increased. If the dynamic pressure is thus increased, the temperature increase of the heat-resistant filter caused by the dynamic pressure increases, so that the filter temperature can be increased.
In the above-described emission control apparatus, a temperature of the exhaust gas may be detected, and the temperature increase need determining device may determine whether there is a need to increase the temperature of the heat-resistant filter based on the detected temperature of the exhaust gas. If the exhaust gas temperature is measured at an upstream side or a downstream side of the heat-resistant filter, it becomes possible to appropriately determine whether there is a need to increase the filter temperature based on the temperature of the filter estimated from the measured exhaust gas temperature. Therefore, this construction is also preferable.
In the emission control apparatus, an inflow temperature of the exhaust gas flowing into the heat-resistant filter may be detected, and the temperature increase need determining device may determine that it is necessary to increase the temperature of the heat-resistant filter, if the inflow temperature detected is lower than a predetermined temperature. If the temperature of exhaust gas flowing into the heat-resistant filter is low, the filter temperature is correspondingly low. Therefore, if the aforementioned determination is made based on the inflow temperature of exhaust gas, it becomes possible to appropriately determine whether there is a need to increase the filter temperature. Thus, this construction is preferable.
Alternatively, an outflow temperature of the exhaust gas flowing out of the heat-resistant filter may be detected, and the temperature increase need determining device may determine that it is necessary to increase the temperature of the heat-resistant filter, if the outflow temperature detected is lower than a predetermined temperature. If the temperature of the heat-resistant filter decreases, the temperature of exhaust gas flowing out of the filter correspondingly decreases. Therefore, if the determination is made based on the outflow temperature of exhaust gas, it becomes possible to appropriately determine whether there is a need to increase the temperature of the heat-resistant filter.
Furthermore, in the emission control apparatus in accordance with the first aspect of the invention, an operation condition of the internal combustion engine may be detected, and the temperature increase need determining device may determine whether there is a need to increase the temperature of the heat-resistant filter based on the detected operation condition. If the operation condition of the internal combustion engine is determined, the exhaust gas temperature is substantially determined, so that the temperature of the heat-resistant filter can be estimated. Therefore, if the determination is made based on the operation condition of the internal combustion engine, it becomes possible to appropriately determine whether there is a need to increase the temperature of the heat-resistant filter. Thus, this construction is preferable.
In the emission control apparatus in which it is determined whether there is a need to increase the temperature of the heat-resistant filter based on the operation condition of the internal combustion engine, the temperature increase need determining device may determine that it is necessary to increase the temperature of the heat-resistant filter, if the internal combustion engine is continuously operated for at least a predetermined time in a predetermined operation region where the temperature of the exhaust gas is lower than or equal to a predetermined temperature. Even if the exhaust gas temperature decreases, the temperature of the heat-resistant filter does not immediately decrease, but starts decreasing after some time in many cases. Therefore, if it is determined that it is necessary to increase the temperature of the heat-resistant filter in the case where the internal combustion engine is continuously operated in the predetermined operation region for at least the predetermined time, it becomes possible to appropriately make the determination. Thus, this construction is preferable.
In the emission control apparatus of the first aspect of the invention, a gas flow resistance of the heat-resistant filter may be detected, and the temperature increase need determining device may determine that it is necessary to increase the temperature of the heat-resistant, if the gas flow resistance detected is greater than a predetermined value. If the rate of removing the carbon-containing particulates collected by the heat-resistant filter is lower than the rate of collecting carbon-containing particulates from exhaust gas, the gas flow resistance of the heat-resistant filter gradually increases due to deposit of particulates. Therefore, if the temperature of the heat-resistant filter is raised by increasing the dynamic pressure on the filter when the gas flow resistance exceeds a predetermined value, the rate of removing the carbon-containing particulates improves, so that the carbon-containing particulates deposited on the filter can be removed. Hence, the gas flow resistance of the filter can be brought back to an appropriate value. Thus, this construction is preferable.
In the emission control apparatus in which the gas flow resistance is detected, the gas flow resistance may be detected by detecting a pressure difference between a pressure in the passage upstream of the heat-resistant filter and a pressure in the passage downstream of the heat-resistant filter. If the gas flow resistance is detected based on the differential pressure across the filter, an accurate gas flow resistance can be detected. Hence, it becomes possible to appropriately determine whether there is a need to increase the temperature of the heat-resistant. Thus, this construction is preferable.
Alternatively, in the emission control apparatus, an operation condition of the internal combustion engine may be detected, and the gas flow resistance may be detected by detecting the pressure in the passage upstream of the heat-resistant filter when the engine is operated under a predetermined operation condition. If the gas flow resistance of the heat-resistant filter is within an appropriate range, the in-passage pressure upstream of the heat-resistant filter automatically becomes a pressure within a predetermined range in accordance with the operation condition of the internal combustion engine. Therefore, if the in-passage pressure is detected upstream of the heat-resistant filter when the internal combustion engine is operated under the predetermined operation condition, it becomes possible to appropriately determine whether there is a need to increase the temperature of the heat-resistant filter, by employing a simple and convenient method. Thus, this construction is preferable.
In the emission control apparatus of the first aspect of the invention, the dynamic pressure on the heat-resistant filter may be increased by advancing a valve opening timing of an exhaust valve provided in a combustion chamber of the internal combustion engine, if it is determined that there is a need to increase the temperature of the heat-resistant filter. If the exhaust valve-opening timing is advanced, exhaust gas with an increased pressure jets out from the combustion chamber via the exhaust valve. Therefore, the flow speed of exhaust gas correspondingly increases. If the flow speed of exhaust gas is increased in this manner, the dynamic pressure on the heat-resistant filter correspondingly increases, so that the temperature of the heat-resistant filter can be increased. Thus, this construction is preferable.
Alternatively, in the emission control apparatus, it is also possible to retard a valve opening timing of an exhaust valve provided in a combustion chamber of the internal combustion engine by at least a predetermined value, instead of advancing the exhaust valve-opening timing, if it is determined that there is a need to increase the temperature of the heat-resistant filter. If the exhaust valve-opening timing is retarded by at least the predetermined value, the exhaust gas in the combustion chamber instantaneously jets out due to the delay in the opening of the exhaust valve, so that the flow speed of exhaust gas increases. As the flow speed of exhaust gas is increased in this manner, the dynamic pressure on the heat-resistant filter correspondingly increases, so that the filter temperature will increase. Thus, this construction is preferable.
In the emission control apparatus of the first aspect of the invention, if the internal combustion engine is operated by injecting a fuel into a combustion chamber of the internal combustion engine and burning the fuel, it is possible to inject an additional fuel during an expansion stroke of the internal combustion engine if it is determined that there is a need to increase the temperature of the heat-resistant filter. If an addition amount of fuel is injected into the combustion chamber during the expansion stroke of the internal combustion engine, the in-combustion chamber pressure at the exhaust valve-opening timing increases due to the combustion of the additionally injected fuel. As a result, the flow speed of exhaust gas increases, so that it becomes possible to increase the dynamic pressure on the heat-resistant filter. Thus, this construction is preferable. Furthermore, the additional fuel injection during the expansion stroke of the engine accomplishes supply of fuel without a considerable effect on the engine output. Therefore, by injecting a slightly increased amount of fuel, the in-combustion chamber pressure occurring at the exhaust valve-opening timing can be effectively increased. Thus, this construction is preferable.
If the emission control apparatus is applied to an internal combustion engine in which fuel is injected into the combustion chamber, a timing of injecting the fuel may be retarded if it is determined that there is a need to increase the temperature of the heat-resistant filter. If the fuel injection timing is retarded, the pressure in the combustion chamber cannot be efficiently converted into output of the internal combustion engine. This means that thermal energy generated by combustion of fuel remains within the combustion chamber without being converted into output of the engine. Therefore, retardation of the fuel injection timing increases the in-combustion chamber pressure occurring at the exhaust valve-opening timing. If the combustion pressure at the exhaust valve-opening timing increases, exhaust gas jets out via the exhaust valve with a correspondingly increased momentum, so that it becomes possible to increase the dynamic pressure on the heat-resistant filter.
Alternatively, if the emission control apparatus of the first aspect of the invention is applied to an internal combustion engine equipped with a turbocharger for compressing air and supplying compressed air into a combustion chamber of the internal combustion engine, a boost pressure of the turbocharger may be increased if it is determined that there is a need to increase the temperature of the heat-resistant filter. If the boost pressure of the turbocharger is increased, the amount of air introduced into the combustion chamber increases, and the amount of exhaust gas correspondingly increases. As a result, the dynamic pressure on the heat-resistant filter can be increased. Thus, this construction is preferable.
Alternatively, if the internal combustion engine has a plurality of combustion chambers, exhaust passages provided separately for the combustion chambers for discharging the exhaust gas from the combustion chambers, and communication passages for interconnecting the exhaust passages in communication, open-close valves may be provided separately for the communication passages, and the open-close valves may be closed if it is determined that there is a need to increase the temperature of the heat-resistant filter. If the open-close valves provided in the communication passages are closed, the entire amount of exhaust gas from a combustion chamber flows only through the exhaust passage of the combustion chamber without any portion of the exhaust gas escaping into another exhaust passage via a communication passage. Therefore, the amount of flow of exhaust gas that passes through the heat-resistant filter increases, so that the dynamic pressure on the heat-resistant filter can be correspondingly increased.
In the foregoing emission control apparatus of the first aspect of the invention, the heat-resistant filter may be a filter as described below. That is, the heat-resistant filter may be a filter which collects the carbon-containing particulates and hydrocarbon compounds contained in the exhaust gas in a dispersed fashion that allows the carbon-containing particulates and the hydrocarbon compounds collected by the filter to contact oxygen present in the exhaust gas, and which allows the carbon-containing particulates and the hydrocarbon compounds collected to burn by using an exhaust gas whose inflow temperature is lower than a combustible temperature of the carbon-containing particulates.
If carbon-containing particulates in exhaust gas are collected in a dispersed fashion by using the above-described filter, the collected particulates can easily be burned. Since this filter makes it possible to easily burn particulates, the combustion of collected carbon-containing particulates can be more effectively accelerated by increasing the dynamic pressure on the filter and thereby raising the filter temperature. Thus, this construction is preferable.
Alternatively, in the emission control apparatus of the first aspect of the invention, the heat-resistant filter may be a heat-resistant filter loaded with an oxidation catalyst. Preferable oxidation catalysts supported on the heat-resistant filter includes precious metal catalysts that exhibit good oxidizing activity and good durability, such as platinum, palladium, rhodium, etc. In the case where the oxidation catalyst-loaded heat-resistant filter is employed, too, an increase in the filter temperature caused by increasing the dynamic pressure on the filter will accelerate the combustion of carbon-containing particulates collected by the filter. Thus, this construction is preferable.